1. Field of the Invention
The invention generally relates to WLAN (Wireless Local Area Network) receivers and corresponding methods, and in particular to techniques for controlling the signal power part in such receivers.
2. Description of the Related Art
A WLAN system is a flexible data communication system that allows a remote user's mobile device to connect to an access point of the network (wired LAN), without having the requirement for the mobile device of being physically attached to the network, as well as to connect to a further remote user device. Thus the mobile device in a WLAN system provides for wireless mobility and additionally achieves the common functionality of wired data transfer as well as application and data access via the network.
As illustrated in FIG. 1, a WLAN system provides connection for mobile devices 140, 150 which are also referred to as (mobile) stations. As indicated by arrows, the mobile devices 140, 150 may communicate with an access point 130 which is connected to a wired network 100. Further the mobile devices 140, 150 can interconnect each other as well. Hence, data transfer between one of the mobile stations 140, 150 and one of wired devices 110, 120 via the access point 130 or another one of the mobile stations 140, 150 can be established.
Presently, Radio Frequency (RF) and Infra Red (IR) transmission techniques are most commonly used in wireless LANs. The industry specification IEEE 802.11 provides a standard for wireless LAN systems and products and describes direct sequence spread spectrum (DSSS) as one possible modulation technique for RF signals. Particularly when using DSSS modulation, but also when using other modulation techniques, a WLAN device internally requires a stable signal level for correctly evaluating a received signal. Hence, a change in the quality of the received signal has to be compensated before further evaluating the signal. For this purpose automatic gain control (AGC) units are provided.
Frequently, AGC units are analog circuits that receive an analog signal and generate a gain control signal for a variable gain amplifier to amplify the signal. An example of a WLAN receiver having an analog AGC unit 210 is shown in FIG. 2 where an analog-to-digital converter (ADC) 220 and a digital signal processing (DSP) unit 230 are located behind the AGC unit 210. However, conventional analog AGC units typically require a complex structure of subunits for operating with sufficient accuracy and fast settling time. Particularly in view of the general desire to reduce the size and power consumption of mobile devices this is a major difficulty of conventional analog AGC units.
Further AGC units may exist that comprise digital units to calculate the AGC control signal. An example is given in FIG. 3 where the AGC unit 310 is located between the ADC unit 300 and the DSP unit 230. Further, there is an amplifier (not shown) located before the ADC unit 300 which needs to be controllable as otherwise the overall performance of the receiver would significantly decrease. The units shown in FIG. 3 however, due to the quantization of their internal values and the corresponding quantization of the AGC control signal, typically add some kind of oscillation to the output signal of the variable amplifier. The oscillation may hinder the correct evaluation of the amplified signal.
Hence, conventional wireless devices, in particular but not exclusively those having an AGC unit, are expensive high quality devices since they need to be built in a complex structure for providing a precise output signal level with fast settling time. However, reducing the requirements with respect to the achievable accuracy and settling time would decrease both system performance and reliability of the WLAN system.